An electrical transmission system comprises protection means arranged to protect, monitor and control the functioning of devices forming part of the transmission system. The protection systems detect, among other things, short-circuits, over-currents and over-voltages in the transmission lines, transformers and other parts of the power transmission system.
Protection equipment is used throughout the electrical power transmission system for providing such protection and control. The protection equipment detects and isolates faults on, for example, transmission and distribution lines by opening circuit breakers, and thereafter restoring power flow after the fault has been eliminated. Alternatively, the protection equipment may be arranged to select an alternative route for the power flow upon detection of a fault.
Current differential protection is a reliable method for detecting faults and thereby protecting the power network. It is based on the idea of comparing currents on both sides of a protected zone, e.g. a transmission line, or a protected unit, e.g. a transformer. Under normal operating conditions, the sum of all currents entering and leaving a protected transmission line is equal to zero.
Uncompensated transmission lines typically range from a few kilometers up to a few hundred kilometers. In order to transmit power along even longer transmission lines, e.g. up to thousands of kilometers, some kind of compensation is needed for compensating for losses and improving the power transfer. Series compensated transmission lines are compensated by means of capacitors, in the following denoted capacitor bank, arranged at a single location along the transmission line.
FIG. 1 illustrates a transmission line system 1 comprising a transmission line 3 between a station A and a station B. The transmission line 3 is series-compensated by means of a capacitor bank 2. Each side of the transmission line is protected by a current differential protective relay, DIFF RELA and DIFF RELB, respectively, in the following denoted simply differential relay. Each side of the transmission line 3 is typically also protected by additional protection devices, not discussed herein.
A fault may occur anywhere on the transmission line between the station A and the station B. In particular, for a series-compensated transmission line, the fault may occur between the station A and the capacitor bank 2, or between the capacitor bank 2 and the station B. A device for locating faults, denoted fault locator 6 (FLA), is arranged in the differential relay DIFF RELA for quickly determining the distance to the fault that has occurred. There are different methods for determining the distance to the fault, and International Patent Application WO 2007/079990, assigned to the same applicant as the present application, discloses one such method for the case of uncompensated transmission lines.
However, to locate faults on series-compensated transmission lines offers several challenges. The capacitance added by the capacitor bank changes the appearance of the location of the fault when using methods successfully applied to uncompensated transmission lines. For example, if the apparent impedance is used as a measure of the distance to a fault, the result is misleading due to the fact that the capacitor bank causes a discontinuity in the transmission line impedance. In particular, the capacitor bank creates a non-linear and current dependent circuit appearing between the substation and the fault location.
It is realized that accurate location of faults on series-compensated transmission lines is very important as such lines usually spread over very large distances, and constitute vital links between the energy production and consumption centres. Different fault location algorithms for series compensated lines have been developed so far, applying one-end or two-end measurements.
Two prior art documents addressing the challenges of locating faults on series-compensated transmission lines include M M Saha, J Izykowski, E Rosolowski, B Kasztenny; “A new accurate fault locating algorithm for series compensated lines”, IEEE Transactions on Power Delivery, Vol. 14, No. 3, July 1999, pp 789-797; and M M Saha, J Izykowski, E Rosolowski, B Kasztenny; “Location of fault on series-compensated power transmission lines”, U.S. Pat. No. 6,529,010, Mar. 4, 2003. However, the methods described in these documents are based on one-end measurements of current and voltages.